The present disclosure relates to technology for controlling driving of an alternating current rotating electric machine.
A high-output alternating current (AC) rotating electric machine used to power an electric automobile, a hybrid automobile, or the like is driven with a high voltage. Since the high-voltage power supply installed in such an automobile is a DC battery, DC power is converted into three-phase alternating current, for example, by an inverter circuit that uses switching elements. The rotating electric machine not only functions as a motor that outputs power for driving the vehicle using electrical energy, but also functions as a generator that generates electricity using kinetic energy from the vehicle, an internal combustion engine, or the like. Electrical power generated by the rotating electric machine is regenerated and stored in a battery.
Incidentally, there are cases where a switchgear (contactor) is provided between the battery and the rotating electric machine, or more specifically between the battery and the inverter. This contactor is a system main relay (SMR) that is constructed using relays, for example, and the contactor enters a conductive state with closed contact points if the vehicle ignition key (IG key) is in the on state (enabled state), and enters a non-conductive state with open contact points if the IG key is in the off state (non-enabled state). In other words, the battery and the inverter (and the rotating electric machine) are electrically connected when the SMR is in the closed state, and the electrical connection between the battery and the inverter (and the rotating electric machine) is cut off when the SMR is in the open state. During normal operation, the open/closed state of the SMR is also controlled according to the state of the IG key. However, even if the IG key is in the on state, there are cases where the SMR is opened due to a vehicle malfunction, collision, or the like. For example, if the supply of power to the SMR is cut off, if an abnormality occurs in the SMR drive circuit, if the SMR has a mechanical malfunction due to vibration, shock, noise, or the like, or if a disconnection occurs in an SMR peripheral circuit, it is possible for the contact points of the SMR to enter the open state, and for the contactor to enter the open state.
For this reason, if the contactor has entered the open state, there are cases where shutdown control (SD control) is implemented to switch all of the switching elements forming the inverter to the off state. A smoothing capacitor (DC link capacitor) for smoothing DC voltage (DC link voltage) is often provided on the DC side (in the DC link portion) of the inverter, and if SD control is implemented, the electrical power stored in the stator coil charges the smoothing capacitor via free wheel diodes (FWDs) that are connected in reverse-parallel to the switching elements. For this reason, it is possible for the voltage across the terminals of the smoothing capacitor (DC link voltage) to rise in a short period of time. Giving the smoothing capacitor a higher capacitance and higher withstand voltage to handle this rise in the DC link voltage leads to an increase in the physical size of the smoothing capacitor. This also requires the inverter to have a higher withstand voltage. This consequently hinders reduction of the size of the rotating electric machine drive device, and also influences parts cost, manufacturing cost, and product cost.
Also, when the contactor has entered the open state, there are cases where active short control (active short circuit control (ASC control)) for switching some of the switching elements to the on state to allow current reflux, such as zero vector sequence control (ZVS control), is executed. For example, JP 2011-55582A discloses a control method in which all of the switching elements on the upper side in the inverter are switched to the off state, and any one or more switching elements on the lower side are switched to the on state (JP 2011-55582A: FIG. 2; paragraphs 158, 159, 165, etc.). Although a rise in the DC link voltage can be suppressed with ASC control, a large current (reflux current) flows through the switching elements and the stator coil. Also, the large current continues to flow until the electrical power stored in the stator coil is consumed through heat or the like. For this reason, there is a possibility of wearing down the switching elements and the stator coil, and reducing their lifetime. Also, switching elements and the like that can handle a large current become necessary, and there is a possibility of also influencing parts cost, manufacturing cost, and product cost.